The invention relates to a method for the evaporation of liquids, especially such liquids which tend to froth when mixed, and to an apparatus for carrying out the method.
The method according to the invention is mainly intended for the pre-evaporation of the waste liquid emerging from a continuous-working digester, such as sulfate black liquor, whereby the concentration of the black liquor is adjusted from approx. 15-18% to approx. 23-25% in film evaporation devices working according to the falling-film principle, using the so-called buffer vapor from the digestion for the evaporation.
In a currently used continuous-working cellulose digester the black liquor passing to the evaporation system is taken out at a pressure of approx. 8 atm (absolute) and at a temperature of approx. 170.degree. C. Thus it contains a considerable amount of thermal energy, which can be exploited in the process.
So far the black liquor has usually first been pre-evaporated by lowering the pressure in two successive expansion stages so that a liquor vapor at approx. 120.degree. C, suitable for the roasting of digestion chips, is obtained from the first stage and that the liquor vapor produced during the second stage is used for heating water. Thereby the liquor reaches a temperature of approx. 100.degree. C, which is considered a suitable inlet temperature for evaporation. The final evaporation is performed in a multistage evaporator based on indirect heat exchange, wherein fresh vapor is fed to the first stage to yield the thermal energy required by the process. This use of heat is not as efficient as it could be.
If expansion vapor from the digester is used as the source of heat for the black liquor evaporation system, the requirement of fresh vapor is reduced. Environmental advantages are also gained.
The use of black liquor expansion vapor for the pre-evaporation is known per se and has been described in U.S. Pat. No. 3,286,763, for example. The object of the invention is to eliminate one problem involved in its application.
The concentration of the black liquor emerging from the digester is usually approx. 15%. If the concentration is less than 20%, the liquor froths easily when mixed or boiled. In evaporation systems based on indirect heat exchange this causes soilage of the heat exchange surfaces and blockage of the flow conduits. In order to eliminate this problem, concentrated liquor from later stages has usually been mixed with the dilute liquor fed to the first stage of the multi-stage evaporator, in a ratio to raise the concentration of the latter to over 20%. This method cannot be used, without worsening the heat economy, in an evaporation system wherein expansion vapor from the digester is used as a source of heat.
From U.S. Pat. No. 2,786,518, for example, a sulfite waste liquor evaporation apparatus is known wherein sulfite waste liquor is brought into an indirect heat exchange contact with a hot medium in a heat exchanger in order to concentrate the waste liquor by evaporation. In this apparatus, part of the concentrated waste liquor is returned and combined with the dilute waste liquor to be evaporated, in order to increase the concentration of the latter before the indirect heat exchange contact. The object of increasing the concentration is to reduce carbon formation on the heat exchange surfaces, which appears when dilute waste liquor is heated.
Such an apparatus is disadvantageous in terms of heat economy because waste liquor flows which are at different temperatures are combined, i.e., a hot return flow is combined with a cooler flow of the waste liquor to be evaporated. In this apparatus the entire liquor quantity to be evaporated is exposed to an indirect heat exchange contact, together with the return liquor.
The object of the invention, on the other hand, is to eliminate the frothing problem without worsening the heat economy. It is especially applicable to black-liquor evaporation systems wherein the pre-evaporation of the liquor is performed in falling-film evaporators, whose working principle has been described in, for example, U.S. Pat. No. 3,366,158.
Thus the object of the present invention is to provide a method and apparatus for the evaporation of liquids, especially liquids which, when dilute, tend to froth during mixing, wherein the liquid to be evaporated is fed into an evaporation zone wherein it is brought into an indirect heat exchange contact with a hot medium in order to concentrate the liquid, and the vaporized portion and concentrated liquid are removed from the evaporation zone.